The first thing to do is figure out how long you plan to be in the house. A future buyer may not want a dark theater and may prefer the door and window. If you are going to be in the house for years then do what you want. You don’t need to remove the door and window, just cover then over with fabric so they look like drapes from the outside, and then build the wall over them. That way it is easy to convert back and you don’t have to spend the money now to reside the house.
The first two things to building a theater are to get it 100% dark and as quiet as possible on your budget. Build the room right once, you can always change gear in the future. Walls that reduce sound transmission lower the noise floor in the room AND lower the volume for people outside the room. Get rid of the can lights, putty the outlets, get a solid door or two, put a dead vent on the HVAC, and build floating walls. There are many manufacturers to help you out, I have used and can recommend
https://www.pac-intl.com/ and
https://www.auralex.com/ . You will also want to do a dimensional analysis of the room so you get the flattest bass response, there are calculators online. You won’t want to lose height or width but you can adjust the length. I would recommend doing it on the wall with the entrance door and make a double wall with double doors. This stuff can be OMG expensive if you go all out, but you can also make large improvements for a moderate cost and it will be better long term then any piece of gear. Do the room right, wire for Atmos and then finish it off as you have time and budget. Also avoid in-ceiling/in-wall speakers unless you back box them with the same level of sound reduction material you use in the rest of the wall.
